What is your zone?

It sounds like you want shrubs that will stay less than two or three feet high, below the windowsill. Does the brick planter have drainage straight down or is it lined?

We are in the Midwest, zone 5. The planter does have drainage holes.

The trick is to find stuff that stays evergreen and needs little pruning . My advice is visit a garden center in your area to see what will provide that with your limitations . Off topic is painting the siding ever been on your list ? I think a dark color like the grout in the brick would be nicer and amke a nice backdrop for plants .Something like this and notice the dark background

I hope an experienced midwestern gardener will have suggestions since zone 5 is outside my experience. Do you have a local garden center that can give advice, perhaps for native shrubs?

IME with older planters, scooping out and replacing all the old dirt with nice fresh dirt mixed with compost and a good fertilizer will get your new plants off to a good start. The old dirt is usually exhausted of nutrients and may have salt buildup as well if it has been used for many years. It's well worth the effort to replace the old with new. It's good that there are drain holes already.

https://www.gardendesign.com/zone/5.html has lots of suggested plants.

It looks as if there's an overhang above the bed. Is there an irrigation mechanism?

Treat this as a large container - it is - and use a container potting mix. For winter interest, you need evergreen. Dwarf conifers would work, tend to be very cold hardy and stay small for a long time.

You will need to water and fertilize regularly, just like you would with a pot type container.

I’m an Iowa native and have been wracking my brain over this problem. A tough planting spot for shrubbery - very small, no direct water source (not even rain), morning sun only, in a bricked structure in Z5.

All the shrubs I can think of need much more room. Exactly how big is this spot, front to back and side to side?

Are you committed to regular watering, most likely daily in June, July, and August?

Would you settle for perennials? There are some that would survive but blooming might be limited by short sunshine periods.